1. Field of the Invention
This invention relates to the chemical arts. In particular, it relates to a method for treating hydrogen sulfide-containing waste gases.
2. Discussion of the Related Art
An important process for removing hazardous hydrogen sulfide (H2S) from various waste gases, including gases produced during the refining of petroleum products, is the modified Claus process. It involves the following net reaction:
H2S+xc2xd02xe2x86x92H20+Sxe2x80x83xe2x80x83(1)
and produces steam and liquid sulfur.
In plants employing the modified Claus process, known as sulfur recovery units or xe2x80x9cSRU""s,xe2x80x9d the liquid sulfur is collected in enclosed pits. The thus recovered liquid sulfur is not pure, but contains residual H2S. The H2S is present not only in the form of H2S dissolved in the liquid sulfur, but in the form of polysulfides (H2Sx). The gradual decomposition of the polysulfides in the liquid sulfur produces additional hydrogen sulfide by the process illustrated in the following equation:
H2Sxxe2x86x92H2S+(xxe2x88x921)Sxe2x80x83xe2x80x83(2)
(dissolved in liquid S) (dissolved in liquid S) (liquid phase)
Methods for purifying the liquid sulfide are known. For example, U.S. Pat. No. 5,632,967, to Nasato, which patent is herein incorporated by reference, describes removing both the H2S and the H2Sx by intimately mixing a stream of the liquid sulfur with a stream of an oxidizing gas and passing the streams through a vessel at a pressure of at least about 40 psig.
However, prior to purification, the dissolved H2S can pass by physical desorption from the liquid sulfur into the vapor space in the enclosed collection pit above the liquid sulfur. This can give rise to potentially dangerous conditions, if the concentration of H2S reaches its lower explosive or flammability limit. (3.4 vol % under normal pit operating conditions.)
Consequently, to prevent fires and explosions, SRU""s generally utilize an air sweep through the vapor space of the collecting pit. The pit sweep air is typically induced into the pit from ambient air by creating a draftxe2x80x94using either a heated natural draft stack, a motor driven blower or a steam driven eductor. The pressure of the gas exiting the blower or eductor is normally about 2 psig.
When the sweep gas leaves the collecting pit it contains H2S, sulfur vapor, and in some instances sulfur dioxide (SO2) produced from the reaction of air with H2S and/or sulfur. It is a drawback of these conventional methods for inducing the sweep gas that they limit subsequent treatment of the gas. For example, the pit vapors are simply discharged into the atmosphere when a natural draft stack is employed. When motor driven blowers or steam driven eductors are used, the sweep gas is normally sent to a thermal incinerator where the sulfur compounds are oxidized to SO2 before discharging into the atmosphere. Consequently, all of these methods result in H2S and/or SO2 being released into atmosphere.
Therefore, there exists a definite need for an improved method of treating sweep gases that minimizes or substantially eliminates the discharge of H2S and/or SO2 into the atmosphere. The present invention satisfies these and other related needs and provides farther related advantages.
Now in accordance with the invention, there has been found a simple, effective, and inexpensive method for treating sweep gases that minimizes or substantially eliminates the discharge of H2S and/or SO2 into the atmosphere. Hydrogen sulfide-containing liquid sulfur, typically produced by a sulfur recovery unit, is introduced into a containment vessel, such as a sulfur collection pit, to partially fill the containment vessel and create a hydrogen sulfide-containing liquid sulfur phase and a hydrogen sulfide-containing vapor phase.
A portion of the hydrogen sulfide-containing liquid sulfur phase is then treated to provide a liquid sulfur phase reduced in or essentially free of hydrogen sulfide and a gaseous hydrogen sulfide-containing phase, such that the gaseous hydrogen sulfide-containing phase has a pressure of at least about 60 psig, preferably from about 80 psig to about 120 psig. In some embodiments, the hydrogen sulfide-containing liquid sulfur phase is withdrawn from the containment vessel and introduced, along with an oxidizing gas, preferably air, into a degassing vessel at a pressure of at least about 60 psig, preferably from about 80 to about 120 psig.
A portion of the hydrogen sulfide-containing vapor phase is then withdrawn from the containment vessel using one or more eductors driven by a motive fluid, where the motive fluid is the gaseous hydrogen sulfide-containing phase. The hydrogen sulfide-containing waste gas stream, exiting the eductor at a pressure of from about 6 to about 14 psi, preferably from about 8 to about 12 psi, is then treated to reduce the hydrogen-sulfide content of the waste gas.
In some embodiments, the waste gas stream exiting one or more of the eductors is used as the motive fluid for an additional eductor prior to treatment of the waste gas stream. Also, in some embodiments, the hydrogen sulfide-containing liquid sulfur is produced by a sulfur recovery unit having a burner for combusting a stream of gas and evolving the resulting combustion products into a modified Claus process reactor furnace and the hydrogen-sulfide containing waste gas is treated by feeding the hydrogen-sulfide containing waste gas into the burner. In other alternative embodiments, the hydrogen sulfide-containing liquid sulfur is produced by a sulfur recovery unit having a Wellman Lord-type tail gas clean-up unit for removing hydrogen sulfide from a gas stream and the hydrogen-sulfide containing waste gas is treated by feeding the hydrogen-sulfide containing waste gas into the Wellman Lord-type tail gas clean-up unit; the hydrogen sulfide-containing liquid sulfur is produced by a sulfur recovery unit having in a sodium bisulfite tail gas clean-up unit for removing hydrogen sulfide from a gas stream and the hydrogen-sulfide containing waste gas is treated by feeding the hydrogen-sulfide containing waste gas into the sodium bisulfite tail gas clean-up unit; the hydrogen sulfide-containing liquid sulfur is produced by a sulfur recovery unit having a direct oxidation catalytic converter for converting hydrogen sulfide to sulfur and the hydrogen-sulfide containing waste gas is treated by feeding the hydrogen-sulfide containing waste gas into the direct oxidation catalytic converter; the hydrogen sulfide-containing liquid sulfur is produced by a sulfur recovery unit having a thermal incinerator converting the hydrogen sulfide to sulfur dioxide and the hydrogen sulfide containing waste gas is treated by feeding the waste gas into the thermal incinerator.